The invention relates to oxidized polymeric carbohydrates and products made thereof. The invention particularly relates to oxidized cellulosic fibrous materials and polysaccharides such as starch.
The cellulosic fibrous material, which is usually known as pulp and which can be used as a starting material in accordance with an embodiment of the invention, is normally used to make paper or nonwoven products. It is mainly composed of cellulose that is obtained from plant cells and that may more or less contain a number of accompanying substances.
The accompanying substances may relate e.g. to lignin, whichxe2x80x94in addition to cellulosexe2x80x94represents a main component in lignified plant cells.
In the case of chemical pulping processes, however, the lignin may be dissolved in an approximately quantitative manner, whereby cellulose and, depending on the pulping process, also hemicelluloses are left behind. As an industrial product that is obtained from lignified plant parts by means of chemical pulping, cellulose is generally referred to as pulp. Cellulose also occurs in a purer form, e.g. in cotton or linters. Usually just purification or removal of fats and waxes is needed to recover the cellulose, because these plant fibers contain next to no lignin.
A paper of an untreated cellulose-containing fibrous material usually loses 95% to 97% of its dry strength when saturated with water, so that it normally cannot be used in the moistened or wet states. This is due to the fact that paper products to an extent develop a dry strength as a result of inter-fiber hydrogen bonds. If the paper product is moistened, the water breaks up the hydrogen bonds and therefore reduces the strength of the paper product.
There are two important techniques for increasing the wet strength of paper products that have already been in use for some time. One technique prevents the water from reaching and breaking up the hydrogen bonds, e.g. by coating the paper product with a water-repellent material. The second approach is to provide the paper product with additives or reagents that promote the formation of inter-fiber bonds.
To increase the wet strength according to the first technique, poly(ethylene imines), polyamide epichlorohydrin resins and urea or melamine formaldehyde condensates are for example used as wet-strength agents. The use of such synthetic resins results in permanent wet strength. On the other hand, however, enhanced wet strength can also be achieved by addition of water-soluble starches or starch derivatives. This effect is nevertheless only temporary and decreases as soon as the starch derivative dissolves. Apart from the aforementioned additives, modified soluble cellulose derivatives are used as wet-strength agents. In this way, for example, the addition of carboxymethyl cellulose is usual as an additive besides the aforementioned polyamide epichlorohydrin resins.
To bond cellulose fibers together according to the second technique, thereby increasing the strength, U.S. Pat. No. 5,873,979 teaches the reaction of the cellulose""s hydroxy functions with a C2-C9 dicarboxylic acid.
Modified techniques for increasing the strength of paper in the wet state are taught in the following applications filed by the Procter and Gamble Company: WO 97/36051, WO 97/36053, WO 97/36037, WO 97/36054 and WO 97/36052.
WO 97/36051 describes paper that exhibits a temporary wet strength and which is obtained by linking a polyhydroxy polymer (e.g. cellulose) to 1,2-disubstituted alkene carboxylic acid, oxidative cleavage of the double bond of the alkene carboxylic acid into aldehyde functions and crosslinking the hydroxy groups of the polyhydroxy polymer with the aldehyde functions.
WO 97/36053 describes a paper product exhibiting temporary wet strength and comprising (a) cellulosic fibers and (b) a binder which has (b-i) a polyaldehyde polymer and (b-ii) a water-soluble polysaccharide with cis-OH groups.
WO 97/36037 describes the aldehyde-modified cellulosic fibers upon which the paper product according to WO 97/36051 is based.
WO 97/36054 discloses a compound that bestows temporary wet strength on paper products and which comprises the ozone oxidation product (containing aldehyde groups) of a polysaccharide in which the OH groups in at least one recurrent unit of the polysaccharide are OH groups in cis position. This document also describes a paper product that comprises cellulosic fibers in combination with the above compound and a method of producing the compounds of the paper product.
WO 97/36052 describes a paper product exhibiting initial wet strength and comprising
(a) cellulosic fibers having free aldehyde groups originating from cellulosic fibers that include a polysaccharide (preferably galactose and/or mannose) in which the OH groups of least a part of the recurrent units are OH groups in cis position, in combination with
(b) a water-soluble polymer having functional groups that can react with the aldehyde groups.
Since cellulose exhibits OH groups in trans position, it is not meant with the polysaccharide defined under item (a) and the hemicellulose portion of pulps that have a high proportion of hemicellulose is to be oxidized and the oxidation product used as a xe2x80x9cbinderxe2x80x9d. Hemicelluloses are derived from (poly)saccharides with OH groups in cis position (e.g. galactose, mannose) that can be rapidly oxidized into aldehyde groups and which can then form (hemi)acetal bonds in accordance with the teaching of this document, such bonds holding the paper product together.
Oxidized starch is known in addition to oxidized cellulose products. For instance, DE-A-197 46 805 discloses a starch that is oxidized at the C6 carbon atom of the glucose units, and discloses its use as a super-absorber, as a seed and fertilizer carrier, as a soil ameliorant, as a material in galenics or as an adhesive and binder. According to U.S. Pat. No. 3,137,588, a starch that contains COOH groups can be obtained by hypochlorite oxidation. After crosslinkage with a metal (Zr, Ti), this produces a water-insoluble coating of cellulosic tissue.
DE-A-1,241,433 and DE-A-1,211,142 both disclose dialdehyde polysaccharides such as dialdehyde starch or cellulose containing aldehyde functions at C2 and C3.
The invention""s object is to improve the strength of carbohydrate networks.
This object is solved by a polymeric carbohydrate with functional groups, these groups relating to aldehyde groups formed at positions C2 and/or C3 as well as at position C6 (corresponding to FIG. 1) of the anhydroglucose units of the polysaccharide chain, as well as by paper and nonwoven (products) obtained thereform such as tissue (products).
The oxidized polysaccharide or polysaccharide-based material of the invention comprises aldehyde groups formed at positions C2 and/or C3 as well as at position C6 of the anhydroglucose units of the polysaccharide chain.
With the term xe2x80x9cpolysaccharidexe2x80x9d it is meant a pure carbohydrate, such as starch or cellulose, whereas xe2x80x9cpolysaccharide-based materialxe2x80x9d refers to materials containing a polysaccharide as major component, such as pulp, the major component of which is xcex1-cellulose. Hereinafter both alternatives will be meant, if not stated otherwise, when using the wording xe2x80x9cpolysaccharidexe2x80x9d.
The wording xe2x80x9cpolysaccharidexe2x80x9d further is intended to distinguish the claimed carbohydrates from oligosaccharides which are typically obtained by condensation of 2 to about 7 monosaccharide units.
According to a preferred embodiment the polysaccharide comprises aldehyde groups at positions C2, C3 and C6 of at least some anhydroglucose units of the polysaccharide chain.
These aldehyde groups are obtained by complete or partial oxidation of the primary and/or secondary hydroxyl groups of the anhydroglucose units so as to produce a polymeric carbohydrate that may also exhibit trialdehyde anhydroglucose units.
In a preferred embodiment, a partial oxidation has taken place rather than oxidation of both secondary OH groups and the primary OH group of an anhydroglucose unit of a carbohydrate chain comprising a plurality of consecutive anhydro sugar units into aldehyde groups.
In a further embodiment, the polysaccharide units are partially not oxidized.
The total content of aldehyde groups of the polysaccharide of the invention is preferably more than 50, more preferably more than 75, in particular more that 100 xcexcmol aldehyde/g, particularly more than 150 xcexcmol aldehyde/g dry weight of the polysaccharide (if not stated otherwise, the term xe2x80x9cdry weightxe2x80x9d means oven-drying at 105xc2x0 C. for 6 hrs until a constant weight is reached). Even greater preference is given to values of more than 200 xcexcmol aldehyde/g, particularly more than 250 xcexcmol aldehyde/g dry weight.
If the polysaccharide-based material has a fibrous structure, such as in pulp, the total content of surface aldehydes is preferably more than 10, 25, 50, 75, 100 and 125 xcexcmol aldehyde/g with increasing preference in this order.
Preferably the total number of aldehyde groups is greater, especially more than 1,5 times greater than the total number of carboxyl groups.
The content of oxidized groups in the polysaccharide according to the invention can be analytically determined as explained in the section xe2x80x9cexamplesxe2x80x9d.
According to the invention it possible that other positions than C2 and/or C3 as well as at position C6 of the anhydroglucose units are also oxidized, though to a lesser extent. Thus a primary oxidation of alcohol groups and a glycolic oxidation (e.g. at C3 and C4) may occur. It is preferred that the content of other aldehyde functions (other than at positions C2 and/or C3 as well as C6) accounts for less than 20%, more preferably less than 10%, in particular less than 5% of the total aldehyde content measured.
According to one aspect, the invention makes available a cellulosic fibrous material, such as pulp, wherein the primary (C6)and secondary (C2 and/or C3) hydroxyl groups of the cellulose chain are at least partially oxidized into aldehyde groups, preferably by means of a combined TEMPO oxidation and periodate oxidation. Such an oxidized cellulosic fibrous material, such as oxidized pulp exhibits heightened initial wet strength, and partially also an increased dry strength (the term xe2x80x9cstrengthxe2x80x9d means in this context the strength of paper or nonwoven formed from the oxidized material; for the measurement of the dry and wet strength please refer to the section xe2x80x9cexamplesxe2x80x9d).
An oxidized cellulosic fibrous material, as preferred embodiment of the invention, preferably displays the following strength parameters:
a breaking length (dry) of at least 1000 m, more preferably at least 1900 m, even more preferably at least 2400 m, even more preferably at least 3000 m, in particular at least 4000 m; and/or
a breaking length (wet) of at least 700 m, more preferably at least 900 m, even more preferably at least 1400 m, in particular at least 1900 m; and/or
a relative wet strength of at least 10%, more preferably at least 20%, even more preferably at least 30%, in particular at least 40%.
All these values refer to the measuring methods explained in the section xe2x80x9cexamplesxe2x80x9d.
Cellulosic fibrous materials, such as pulp, normally do not exhibit any initial wet strength. This property may be achieved by selective oxidation of the cellulosic fibrous material due to the introduction of aldehyde groups without essential cleavage of the carbohydrate chain. Thus modified pulps exhibit an initial wet strength.
Particular suitability for oxidation is exhibited by polymers which, like cellulose, are composed of xcex2-1-4 glycosidically bound anhydroglucose units or polymers which are predominantly composed of xcex1-1-4 glycosidically bound anhydroglucose units. xcex2-1-4 polyglycosides composed of anhydroglucose units are usually referred to as cellulose, whereas xcex1-1-4 polyglycosides composed of anhydroglucose units are known as starch. These differ at least in terms of the properties of dissolving in aqueous solutions. Whereas cellulose is not usually soluble in water, starch can be dissolved in aqueous solutions, depending on the molecular weight.
A further aspect makes available a polysaccharide, particularly starch, whose primary (C6) and secondary (C2 and/or C3) hydroxyl groups are oxidized into aldehyde groups at least in part. Such oxidations can be achieved in different ways, particularly by means of xe2x80x9cTEMPO oxidationxe2x80x9d at position C6 and periodate oxidation (xe2x80x9cperiodate cleavagexe2x80x9d) at C2/C3.
It may be advantageous, particularly during the oxidation of starch, to oxidize just a specific part of the anhydroglucose units both at the primary and at the secondary alcoholic groups.
As illustrated in Formula 2, the invention may relate to a trialdehyde, but the invention also includes the oxidation of the hydroxymethyl group (C6 of the anhydroglucose unit) into a carboxyl group, as shown in Formula 3. The invention also comprises the formation of a carboxyl group at the primary alcoholic group.
The formation of individual carboxyl groups at C2 or C3 may be advantageous in the case if starch, because a so-called xcex2-elimination reaction that leads to a cleavage of the anhydroglucose unit occurs in an alkaline medium. This is accompanied by the decrease in the degree of polymerization and by easier solubility in aqueous solvents. 
The starting pulps which may be used for oxidation may relate to primary fibrous materials (raw pulps) or to secondary fibrous materials, whereby a secondary fibrous material is defined as a fibrous raw material recovered from a recycling process. The primary fibrous materials may relate both to a chemically digested pulp and to mechanical pulp such as thermorefiner mechanical pulp (TMP), chemothermorefiner mechanical pulp (CTMP) or high temperature chemithermomechanical pulp (HTCTMP). Synthetic cellulose-containing fibers can also be used. Preference is nevertheless given to the use of pulp from plant material, particularly wood-forming plants. Fibers of softwood (usually originating from conifers), hardwood (usually originating from deciduous trees) or from cotton linters can be used for example. Fibers from esparto (alfa) grass, bagasse (cereal straw, rice straw, bamboo, hemp), kemp fibers, flax and other woody and cellulosic fiber sources can also be used as raw materials. The corresponding fiber source is chosen in accordance with the desired properties of the end product in a manner known in the art. For example, the fibers present in softwood which are shorter than those of hardwood lend the final product a higher stability on account of the higher diameter/length ratio. If the softness of the product is to be promoted, which is important e.g. for tissue paper, eucalyptus wood is particularly suitable as a fiber source.
With regard to the softness of the products, the use of chemical raw pulps is also preferred, whereby it is possible to use completely bleached, partially bleached and unbleached fibers. The chemical raw pulps suitable according to the invention include, inter alia, sulfite pulps, kraft pulps (sulfate process), soda pulps (cooking with sodium hydroxide), pulps from high-pressure cooking with organic solvents (e.g. Organosolv, Organocell, Acetosolv, Alcell) and pulps from modified processes (e.g. ASAM, Stora or Sivola process). Among the kraft pulps, it is possible to use those which were obtained in continuous cooking systems (MCC (modified continuous cooking), EMCC (extended modified continuous cooking) and ITC (isothermal cooking)). The products of discontinuous kraft processes (e.g. RDH (rapid displacement heating), Superbatch and Enerbatch) are also suitable as a starting product. The sulfite processes include the acidic sulfite/bisulfite processes, bisulfite process, xe2x80x9cneutral sulfite semi-chemical pulpingxe2x80x9d (NSSC) process and alkaline sulfite processes such as processes in which in addition to aqueous alkali, sulfite and/or anthraquinone in combination with organic solvents such as methanol were used for cooking, e.g. the so-called ASAM process (alkali sulfite anthraquinone methanol). The major difference between the acidic and neutral or alkaline sulfite processes is the higher degree of delignification in acidic cooking processes (lower kappa numbers). The NSSC process provides semi-chemical pulps which are advantageously defibered in downstream mechanical fibrillation before they are used according to the invention for the purpose of oxidation. The sulfite and kraft pulps considerably differ in terms of their fibrous material properties. The individual fiber strengths of sulfite pulps are usually much lower than those of kraft pulps. The mean pore width of the swollen fibers is also greater in sulfite pulps and the density of the cell wall is lower compared to sulfate pulps, which simultaneously means that the cell-wall volume is greater in sulfite pulps. For this reason, there are also obvious differences regarding water absorption and swelling behavior of the cellulosic fibrous materials, which must also be taken into consideration when selecting the raw material for oxidation.
Before a raw pulp is oxidized in the process according to the invention, it may also be advantageous to allow further delignification to occur in a separate process step. In this way, for example, pulps which
were cooked in alkaline pulp production techniques such as the kraft process or ASAM process
were cooked in acidic pulp production techniques such as the acidic magnesium bisulfite process, and/or
originate from processes in which organic solvents such as methanol (e.g. Organosolv, Organocell, Acetosolv, Alcell) were used should undergo a bleaching process to achieve a more extensive removal of lignin after the cooking process and to obtain a completely cooked pulp.
It is also preferred that before oxidation, the chemical pulp should undergo additional surface treatment (beating) which has a favorable effect on the strength properties of the obtained paper/tissue paper/nonwoven product.
Oxidation is preferably performed so as to first oxidize primary hydroxyl groups (at C6) of the general type H2R-OH into aldehyde groups. This can be selectively achieved by using a sterically hindered organic nitroxy compound. One, particularly two bulky groups in the xcex1 position to the NO is/are suitable for sterical hindrance of the NO group, e.g. optionally substituted phenyl or aliphatic substituents that are linked to the nitrogen atom of the NO by a quaternary C atom, e.g. tert-butyl. Two substituents can also be combined into an alkenyl unit optionally interrupted by a hetero-atom (e.g. O,N) (to form an alicyclic or heterocyclic ring).
Oxidation with nitroxy compounds usually leads, via the aldehyde function, to the carboxy function, the choice of oxidizing system and oxidation conditions making it possible to control the reaction such as to isolate the aldehyde stage. The proportion of carboxy groups usually rises as the reaction duration increases.
Preferred oxidation systems can be represented by the following formula IV 
where n=0 or 1 and where the methylene groups of the ring may carry one or more substituents selected from alkyl, alkoxy, aryl, aryloxy, amino, amido (e.g. acetamido, 2-bromacetamido and 2-iodacetamido), oxo, cyano, hydroxy, carboxyl, phosphonooxy, maleimido, isothiocyanato, alkyloxy, flyorophosphinyloxy (particularly ethoxyfluorophosphinlyoxy), substituted or unsubstituted benzoyloxy, e.g. 4-nitrobenzoyloxy. If n=1 (i.e. the ring represents a piperidine), these groups preferably substitute the 4-position of the piperidine. The di-tert.-alkyl nitroxy unit can also be present as part of a polymer structure such as {(CH3)2xe2x80x94Cxe2x80x94(CH2)2-3xe2x80x94(CH3)2xe2x80x94Cxe2x80x94NOxe2x80x94}mxe2x80x94. Hydroxy, amino and amido are preferred among these substituents on account of the stability of the nitroxy compound under acidic conditions.
An example of n=0 is PROXYL (2,2,5,5-tetramethylpyrrolidine-N-oxyl).
Among the formula (IV) compounds, the case of n=1 is preferred. This leads to the preferred optionally substituted TEMPO compounds (2,2,6,6-tetramethyl-4-piperidine-N-oxide) which can selectively oxidize the primary hydroxy group at C(6) of the anhydroglucose unit of the polysaccharide groups.
The nitroxy compound such as optionally substituted TEMPO can be used
1) catalytically in combination with a primary oxidizing agent capable of returning the nitroxy compound to the oxidized state after its consumption (reduction), or
2) with the nitroxy compound as the only oxidizing agent (in the literature sometimes referred to as xe2x80x9cstoichiometric reactionxe2x80x9d, although according to the invention it is not necessary to oxidize all hydroxy functions of the polysaccharide).
A list of suitable reaction conditions and compounds is found in A. E. J. De Nooy, Synthesis 1996, 1153-1174.
The pH range of the reaction generally varies between 1 and 14, preferably 2 and 7, particularly 3 and 6, apart from exceptions mentioned below. The reaction temperature is preferably between 5xc2x0 C. and 80xc2x0 C. The nitroxy compound may be added to a solution or suspension of the polysaccharide depending on its solubility in the aqueous reaction medium. It can be added as a solid (also as a pasty substance) or as a solution (usually as an aqueous solution). The preferred reaction time ranges from 15 min to several hours.
In the first case (nitroxy compound+primary oxidizing agent), preferably first the nitroxy compound and then the primary oxidizing agent is added. The oxidizing agent can be added all at once or distributed over the duration of the reaction (e.g. by evenly adding it in drops). The primary oxidizing agent is preferably selected from peracids, such as peracetic acid or perboric acid; ozone; hypohalite e.g. NaOCl or KOCl; hypochlorous acid; metal compounds that contain oxygen, such as a suitable oxidic compound of a metal of the transition metals of periods Va to VIIIa in the oxidation state of at least +3, e.g. oxides and oxygen-containing ions of manganese, chromium, iron, nickel, ruthenium and vanadium, e.g. vanadium pentoxide, iron oxide, chromium (VI) oxide, chromates and particularly manganese (IV) oxide and salts of permanganic acid; or oxidizing enzymes).
It is preferred to use chlorine-free oxidizing agents that contain oxygen, such as ozone or peracetic acid. The peracetic acid may be so-called equilibrium peracetic acid that represents an equilibrium of hydrogen peroxide and peracetic acid; preference is given to the use of distilled peracetic acid that contains only minute quantities of free hydrogen peroxide. In the case of oxidation using peracetic acid, it is also beneficial to add a water-soluble bromide such as sodium bromide or bromoacetic acid. There is no intention, however, to exclude the implementation of oxidation without the use of bromine.
The primary oxidizing agent is preferably used in an amount of 0.1 to 20 wt. %, particularly 0.5 to 7.5 wt. %, in relation to the dry weight (oven-dried) of the polysaccharide. The catalytic quantity of the nitroxy compound is preferably 0.05 to 15 wt. % relative to the dry weight (oven-dried) of the polysaccharide.
If the nitroxy compound is the only oxidant (above case 2), this compound is preferably used in amounts up to 150 wt % with respect to the dry weight of the polysaccharide, e.g. pulp.
Further preferred embodiments to perform the oxidation polysaccharide, e.g. pulp at position C(6) of the anhydroglucose units of the polysaccharide to the corresponding aldehyde are explained in the following.
One preferred version lies in the oxidation of the polysaccharide, e.g. pulp with a catalytic amount of the formula I nitroxy compound that is hydroxy-, amino- or amido-substituted (e.g. 4-hydroxy TEMPO) at a pH of less than 9, more preferably between 1 and 7, particularly 2 to 6. In this version, a hypohalite (e.g. NaOCl) or ozone is particularly suitable as a primary oxidizing agent. The nitroxy compound is preferably used here in an amount of 0.05 to 15 wt. % and the primary oxidizing agent in an amount of 0.1 to 20 wt. %, each relative to the dry weight of the polysaccharide. The reaction temperature is preferably 5 to 50xc2x0 C. Depending on the reaction duration, this embodiment of oxidation results in the desired formation of aldehydes at C(6) of anhydroglucose units of the polysaccharide. Halogen-free acids, such as sulfuric acid or toluene sulfonic acid, are particularly suitable for setting the pH.
The preferred (sole) formation of aldehyde groups can be influenced by the pH at which oxidation is performed and by the use of 4-OH TEMPO. In general, oxidation of cellulosic fibrous materials such as pulp is performed in an aqueous suspension, i.e. in a heterogeneous phase. A pH of  less than 9, preferably of 7-1, is set. In this way, aldehyde groups are formed particularly in the acidic pH range when 4-OH TEMPO is used, without any carboxyl groups being formed to a substantial extent.
Further TEMPO and in particular amido-substituted nitroxy compounds (e.g. 4-acetamido TEMPO) of the aforementioned formula I are suitable for stoichiometric oxidation at a pH between 1 and 7, particularly 2 to 6, particularly  less than 3, e.g. 2 to  less than 3. Such low pH shifts the equilibrium between disproportionation and synproportionation further toward disproportionation. Thus it is particularly beneficial to perform the reaction in a pH range  less than pH 3. In this reaction, HCl or halogen-free acids such as sulfuric acid or toluene sulfonic acid are particularly suitable for adjusting the pH. Under these conditions, the nitroxy compound present as a radical disproportionates into an oxammonium ion that acts as an oxidizing species and a hydroxylamine. After the reaction, the consumed (reduced) form of the nitroxy compound can be regenerated with ozone or another oxidizing agent, preferably in a separate process step. An important advantage of the oxidation version discussed here is the ability to use the choice of a suitable pulp (TCF, see following sections) to conduct the entire paper/tissue-paper/nonwoven production method without any halogen-containing chemicals.
Particular suitability for oxidation is exhibited by 4-hydroxy TEMPO. The amount of 4-hydroxy TEMPO used varies between 150-0.05% relative to the polysaccharide (dry weight). The 4-hydroxy TEMPO should be added in a molar ratio of 2:1, if no further oxidizing agent is added in order to achieve complete oxidation of the primary alcoholic group. 4-acetamido. TEMPO can also be used, however, in equal molar amounts without any further oxidizing agent in order to oxidize the primary alcoholic group.
The oxidation reaction is performed such as to adjust the aqueous suspension of cellulose first to a consistency of 20-0.1%, preferably 10-0.1%. The acidic pH range is adjusted by addition of hydrochloric acid (HCl), sulfuric acid (H2SO4), toluene sulfonic acid, phosphoric acid (H3PO4), nitric acid (HNO3) or acetic acid (CH3COOH). In the case of chlorine-free production processes, the use of hydrochloric acid is preferably dispensed with. The N oxide can then be added. It is irrelevant here whether the N oxide is previously dissolved and then incorporated into the aqueous suspension or whether it is added as a solid. Due to the ready solubility of 4-hydroxy-2,2,6,6,-tetramethyl piperidine oxide in water, complete dissolution may be present, even if it is used in excess of a stoichiometric amount.
After the reaction of the polysaccharide, for instance a cellulosic fibrous material, such as pulp, this material is preferably washed or otherwise purified so that there is a quantitative removal of the chemicals used.
This is preferably followed by oxidation of the secondary alcoholic groups, a periodate, in particular sodium periodate preferably being used as an oxidizing agent. Alternatively lead tetraacetate as well as NaOCl, NaOBr and the corresponding acids could be used.
There is no intention, however, to exclude the fact that the pulp or another polysaccharide is first oxidized with periodate and then the TEMPO oxidation is performed.
Depending on the number of secondary alcoholic groups converted at the polymer into aldehyde groups, the use of varyingly large amounts of periodate, preferably sodium periodate is necessary. Preferably an amount of 1 to 2.5 wt. %, based on the dry weight of the polysaccharide, in particular cellulosic fibrous material, is used.
If all the secondary alcoholic groups are to be completely oxidized, a stoichiometric ratio of periodate to secondary OH groups of 1:1 is needed.
The reaction is conducted at a pH of  less than 7, the use of a pH range of 2-6, more preferably 3-5.5, in particular 4-5 being advantageous. After adjustment of the pH, the periodate is dissolved in the aqueous medium, containing the polysaccharide, e.g. the cellulosic fibrous material suspended in an aqueous medium, and is reacted with the polysaccharide by being stirred. The reaction time may vary between a few minutes and several days. Preferably it ranges from 10 min. to 1 day. A temperature of between 1 and 50xc2x0 C. can be chosen for the reaction; oxidation can usually occur at room temperature.
After the reaction the oxidized polysaccharide is purified. Water can be used to wash the chemicals out of the cellulosic fibrous material.
Further it can be preferred to recover the periodate after use since it is an expensive chemical. This can be achieved electrochemically or chemically, e.g. with NaOCl.
A polysaccharide oxidized both at the primary and at the secondary alcoholic groups enjoys advantages over the non-oxidized product.
In the case of the oxidation of pulps, the introduction of aldehyde groups at positions C2 and/or C3 as well as C6, in particular C2, C3 and C6 of the anhydroglucose units results in a higher initial wet strength, partially also dry strength than can alone be achieved by just oxidizing position C6 (TEMPO oxidation) or alternatively the two OH groups at C2 and C3 (periodate oxidation).
According to the present knowledge the best mode to perform the invention involves
1. first oxidizing a cellulosic fibrous material, such as pulp in an aqueous system, by means of 4-Hydroxy-TEMPO or 4-Acetamido-TEMPO in an amount of 0,5 to 10 wt. %, based on the dry weight of the fibrous material, in the presence of NaOCl in an amount of 0,1 to 5 wt. %, based on the dry weight of the fibrous material at a pH of 2 to 10 and a temperature of 5 to 60xc2x0 C. over a time period of 0,25 to 3 h, in order to generate aldehyde functions at position C(6) of the cellulosic chains within the material;
2. then filtering off and washing the oxidized cellulosic fibrous material,
3. then further oxidizing the cellulosic fibrous material using sodium periodate in an amount of 1 to 25 wt. %, based on the dry weight of the fibrous material at a pH of 2 to 6 and a temperature of 10 to 60xc2x0 C. over a time period of 15 min to 1 day, in order to generate aldehyde functions at position C(2) and (3) of the cellulosic chains within the fibrous material.
The present invention also relates to paper or nonwoven (products) comprising the oxidized polysaccharide, in particular cellulosic fibrous material according to the invention, preferably in the amount of at least 50 wt. -%, especially at least 80 wt.-% cellulosic fibrous material, relative to the finished paper or nonwoven (product) (dry weight).
Preferably, the paper is a tissue paper.
The tissue paper or nonwoven may be one-ply or multiple-ply.
The German terms xe2x80x9cVliesxe2x80x9d and xe2x80x9cVliesstoffexe2x80x9d (English: nonwoven) are applied to a wide range of products which in terms of their properties are located between the groups, paper, cardboard, and board on the one hand and the textile products on the other, and are currently summarized under the term xe2x80x9cnonwovensxe2x80x9d (see ISO 9092-EN 29092). The invention allows the application of a large number of extremely varied processes for producing nonwovens, such as what are called air-laid and spun-laid techniques, as well as wet-laid techniques.
Nonwovens may also be called textile-like composite materials, which represent flexible porous fabrics that are not produced by the classic methods of weaving warp and weft or by looping, but by intertwining and/or by cohesive and/or adhesive bonding of typical synthetic textile fibers which may for example be present in the form of endless threads or threads prefabricated of an endless length, as synthetic threads produced in situ or in the form of staple fibers. The nonwovens according to the invention can thus consist of mixtures of synthetic fibers in the form of stable fibers and the fibrous material according to the invention.
xe2x80x9cPapersxe2x80x9d are also planar materials, albeit essentially composed of fibers of a vegetable origin and formed by drainage of a fibrous-material suspension on a screen or between two continuously revolving screens and by subsequent compression and drainage or drying of the thus produced fibrous mat (cf. DIN 6730, May 1996). The standard restricts the range of mass per unit area (basis weight) for paper to a maximum of 225 g/m2.
Depending on the type of paper, the production process also comprises a gluing and/or smoothing step, along with the typical process steps of sheet formation, pressing, and drying described above.
Based on the underlying compatibility of the production processes (wet laying), xe2x80x9ctissuexe2x80x9d production is counted among the paper making techniques. The production of tissue, or more accurately, raw tissuexe2x80x94meaning the one-ply (intermediate) product manufactured on a special-purpose tissue machine or tissue paper making machinexe2x80x94is distinguished from paper production by its extremely low basis weight of normally not more than 65, in particular not more than 40 g/m2 and its much higher tensile energy absorption index (In processing inventive pulp to tissue paper or tissue paper products (single-ply or multi-ply), one generally selects a basis weight of 8 to 65 g/m2, especially 10 to 40 g/m2.) The tensile energy absorption index is arrived at from the tensile energy absorption in which the tensile energy absorption is related to the test sample volume before inspection (length, width, thickness of sample between the clamps before tensile load). Paper and tissue paper also differ in general with regard to the modulus of elasticity that characterizes the stress-strain properties of these planar products as a material parameter.
A tissue""s high tensile energy absorption index results from the outer or inner creping. The former can be produced by compression of the paper web adhering to a dry cylinder as a result of the action of a crepe doctor or in the latter instance as a result of a difference in speed between two screens (xe2x80x9cfabricsxe2x80x9d). This causes the still moist, plastically deformable paper web to be internally broken up by compression and shearing, thereby rendering it more stretchable under load than an uncreped paper. Most of the functional properties typical of tissue and tissue products result from the high tensile energy absorption (see DIN EN 12625-4 and DIN EN 12625-5).
An example of papers and paper products is represented by hygiene papers, particularly tissue papers and hygiene products (tissue products) made therefrom and which are e.g. used in personal grooming and hygiene, the household sector, industry, the institutional field in a wide variety of cleaning processes. They are used to absorb fluids, for decorative purposes, for packaging or even as supporting material, as is common for example in medical practices or in hospitals.
Hygiene paper primarily includes all kinds of dry-creped tissue paper, as well as wet-creped paper and cellulose or pulp wadding.
The one-ply intermediate products originating from the paper-making machine and made of lightweight paper usually dry-creped on a yankee cylinder by means of a crepe doctor are generally described as xe2x80x9ctissue paperxe2x80x9d or more accurately raw tissue paper. The one-ply raw tissue may be built up of one or a plurality of layers respectively.
All one-ply or multi-ply final products made of raw tissue and tailored to the end user""s needs, i.e. fabricated with a wide variety of requirements in mind, are known as xe2x80x9ctissue productsxe2x80x9d.
Typical properties of tissue paper include the ready ability to absorb tensile stress energy, their drapability, good textile-like flexibility, properties which are frequently referred to as crumple softness, a high surface softness, a high specific volume with a perceptible thickness, as high a liquid absorbency as possible and, depending on the application, a suitable wet and dry strength as well as an interesting visual appearance of the outer product surface. These properties allow tissue paper products to be used for example as cleaning cloths, sanitary products (e.g. toilet paper), paper handkerchiefs, cosmetic wipes (facials) or as serviettes/napkins.
Paper can be formed by placing the fibers, in an oriented or random manner, on one or between two continuously revolving screens of a paper making machine while simultaneously removing the main quantity of water of dilution until dry-solids contents of usually between 12 and 35% are obtained.
If tissue paper is to be made out of the cellulosic fibrous material according to the invention, the process essentially comprises
a forming that includes the headbox and the screen portion,
b the drying portion (TAD (through air drying) or conventional drying on the yankee cylinder) that also usually includes the crepe process essential for tissues,
c the monitoring and winding area.
Drying the formed primary fibrous web occurs in one or more steps by mechanical and thermal means until a final dry-solids content of usually about 93 to 97%. In the case of tissue making, this stage is followed by the crepe process which crucially influences the properties of the finished tissue product in conventional processes. The dry crepe process that is predominantly in use today involves creping on a usually 4.5 to 6 m diameter drying cylinder, the so-called yankee cylinder, by means of a crepe doctor in the case of the aforementioned final dry-solids content of the raw tissue paper (wet creping can be used if lower demands are made of the tissue quality). In the next step, the creped, finally dry raw tissue paper (raw tissue) is then usually wound on to a supporting core to form a so-called reel drum or is wound in a longitudinally cut manner on cores to form master rolls and is available in this form for further processing into the paper product or tissue paper product according to the invention.
Instead of the conventional tissue making process described above, the invention gives preference to the use of a modified technique in which an improvement in specific volume is achieved by a special kind of drying within process section b and in this way an improvement in the bulk softness of the thus made tissue paper is achieved. This known process, which exists in a variety of subtypes, is termed the TAD (through air drying) technique. It is characterized by the fact that the xe2x80x9cprimaryxe2x80x9d fibrous web that leaves the sheet making stage is pre-dried (like a nonwoven) to a dry-solids content of about 80% before final contact drying on the Yankee cylinder by blowing hot air through the nonwoven fabric. The nonwoven fabric is supported by an air-permeable screen or belt and during its transport is guided over the surface of an air-permeable rotating cylinder drum. Structuring the supporting screen or belt makes it possible to produce any pattern of compressed zones broken up by deformation in the moist state, resulting in increased mean specific volumes and consequently leading to an increase in bulk softness without decisively decreasing the strength of the fiber web fabric. Another possible influence on the softness and strength of the raw tissue lies in the production of a layering in which the primary nonwoven fabric to be formed is built up by a specially constructed headbox in the form of physically different layers of fibrous material, these layers being jointly supplied as a pulp strand to the sheet making stage. The surface softness can be significantly increased by suitably selecting the raw materials in the headbox outlet nozzle channels that determine the layering, for example by using eucalyptus fibers and/or wattle, at the nonwoven side facing toward the yankee cylinder surface, which benefits the products manufactured from raw tissue production.
When processing the raw nonwoven or raw paper into the final product (third process section), the following procedural steps are normally used individually or in combination: cutting to size (longitudinally and/or cross cutting), producing a plurality of plies, producing mechanical ply adhesion, volumetric and structural embossing, ply adhesion, folding, imprinting, perforating, application of lotions, smoothing, stacking, rolling up.
To produce multi-ply tissue papers, such as handkerchiefs, toilet paper, towels or kitchen towels, an intermediate step preferably occurs with so-called doubling in which the raw tissue is usually unwound in a reel drum number corresponding to the finished product""s desired number of plies and is wound on a common multiply master roll. This processing step frequently includes smoothing or calendering in two-roll or multi-roll calenders. After completion of drying and creping, smoothing (calendering) may also, however, be performed in the tissue machine directly before reeling up.
The processing step from the raw tissue that has already been optionally wound up in several plies to the finished tissue product occurs in special purpose-built processing machines which include operations such as repeated smoothing of the tissue, edge embossing, to an extent combined with planar and/or local gluing to produce ply adhesion of the individual plies (raw tissue) to be combined together, as well as longitudinal cut, folding, cross cut, placement and bringing together a plurality of individual tissues and their packaging as well as bringing them together to form larger surrounding packaging or bundles. Instead of edge embossing, the production of ply adhesion can also be achieved by edging, as is common e.g. in the case of cosmetic tissues. The individual paper ply webs can also be pre-embossed and then combined in a roll gap according to the foot-to-foot or nested methods.